Ethylene oxide catalyst and process

ABSTRACT

This invention relates to an ethylene oxide catalyst having improved selectivity stability which catalyst comprises silver, a promoting amount of alkali metal, a promoting amount of rare earth, a promoting amount of rhenium and, optionally, a promoting amount of rhenium co-promoter selected from sulfur, molybdenum, tungsten, chromium and mixtures thereof, supported on a porous refractory support.

This is a continuation division, of application Ser. No. 064,287, filedMay 17, 1993, now U.S. Pat. No. 5,447,897.

FIELD OF THE INVENTION

The invention relates to silver-containing catalysts suitable for thepreparation of ethylene oxide and to the use of the catalysts for thepreparation of ethylene oxide.

BACKGROUND OF THE INVENTION

Catalysts for the production of ethylene oxide from ethylene andmolecular oxygen are generally supported silver catalysts. Suchcatalysts are typically promoted with alkali metals. The use of smallamounts of the alkali metals potassium, rubidium and cesium were notedas useful promoters in supported silver catalysts in U.S. Pat. No.3,962,136, issued Jun. 8, 1976, and U.S. Pat. No. 4,010,115, issued Mar.1, 977. The use of other co-promoters, such as rhenium, or rhenium alongwith sulfur, molybdenum, tungsten and chromium is disclosed in U.S. Pat.No. 4,766,105, issued Aug. 23, 1988, and U.S. Pat. No. 4,808,738, issuedFeb. 28, 1989. U.S. Pat. No. 4,908,343, issued Mar. 13, 1990, disclosesa supported silver catalyst containing a mixture of a cesium salt andone or more alkali metal and alkaline earth metal salts.

U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285issued Jun. 8, 1976 and British Patent No. 1,325,715, published Aug. 8,1973, disclose the use of silver-rhenium ethylene oxide catalysts. Inthese patents a high surface area silver derivative such as silver oxideis impregnated with a rhenium solution and subsequently reduced toprovide metallic rhenium alloyed with the silver. The '285 patentdiscloses the use of KOH to precipitate Ag₂ O from AgNO₃. There is nodisclosure in the patents of the use of suitable inert supports such asporous refractory supports. U.S. Pat. 4,548,921, issued Oct. 22, 1985,discloses the use of rhenium in silver-supported ethylene oxidecatalysts. In this reference, the rhenium is first placed on the supportin the form of finely divided metal particles and the silver issubsequently deposited on the outer surface of the particles. U.S. Pat.No. 3,316,279, issued Apr. 25, 1967, discloses the use of rheniumcompounds, particularly ammonium and alkali metal perrhenates for theoxidation of olefins to olefin oxides. In this reference, however, therhenium compounds are used, unsupported, along with a reaction modifier(cyanides, pyridines or quinolines) in a liquid phase reaction. U.S.Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method fordistributing catalytically active metallic components on supports usingan impregnating solution of catalyst precursor compound and an organicthioacid or a mercaptocarboxylic acid. Catalytically active metalsinclude metals of Groups IVA, IB, VIB, VIIB and VIII, including rheniumand which may be in either the oxidized or reduced state. However,promoting amounts of rare earth in combination with silver and promoteramounts of alkali metal and rhenium on a porous refractory support arenot suggested. U.S. Pat. No. 4,459,372, issued Jul. 10, 1984, disclosesthe use of rhenium metal in combination with a surface metallated (usingTi, Zr, Hf, V, Sb, Pb, Ta, Nb, Ge and/or Si) alumina or silica. U.S.Pat. No. 4,005,049, issued Jan. 25, 1977, teaches the preparation of asilver/transition metal catalyst useful in oxidation reactions. In thisinstance, the silver serves as both a catalyst and a support for thetransition metal co-catalyst. In U.S. Pat. No. 4,536,482, issued Aug.20, 1985, catalytically active metals such as Ag and Re are co-sputteredalong with a co-sputtered support material on a particular support. U.S.Pat. No. 4,257,976, issued Mar. 24, 1981, discloses a catalystcombination of reduced silver, a carbonate of a rare earth metal andyttrium, a salt of an alkali or alkaline earth metal and a catalystcarrier. None of these references disclose the use of a promoting amountof rare earth which is present on a silver-based, alkalimetal/rhenium-doped supported catalyst.

SUMMARY OF THE INVENTION

The invention relates to a catalyst for the production of ethylene oxidecatalyst from ethylene and molecular oxygen in the vapor phase whichcatalyst comprises a catalytically effective amount of silver, apromoting amount of alkali metal, a promoting amount of a rare earthmetal compound, a promoting amount of rhenium and optionally, a rheniumco-promoter selected from sulfur, molybdenum, tungsten, chromium andmixtures thereof supported on a porous, refractory support.

It has been found that catalysts containing a promoting amount of rareearth compound have higher selectivity stabilities than those obtainedwith catalysts containing no rare earth compound.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, in the vapor phase reaction of ethylene with oxygen toproduce ethylene oxide, the ethylene is present in at least a doubleamount (on a molar basis) compared with oxygen, but frequently is oftenmuch higher. Therefore, the conversion is calculated according to themole percentage of oxygen which has been consumed in the reaction toform ethylene oxide and any oxygenated by-products. The oxygenconversion is dependent on the reaction temperature, and the reactiontemperature is a measure of the activity of the catalyst employed. Thevalue T40 indicates the temperature at 40 mole percent oxygen conversionin the reactor and the value T is expressed in ° C. This temperature forany given catalyst is higher when the conversion of oxygen is higher.Moreover, this temperature is strongly dependent on the employedcatalyst and the reaction conditions. The selectivity (to ethyleneoxide) indicates the molar amount of ethylene oxide in the reactionproduct compared with the total molar amount of ethylene converted. Inthis specification, the selectivity is indicated as S₄₀, which means theselectivity at 40 mole percent oxygen conversion. The selectivity ofsilver-based ethylene oxide catalysts can and will decrease over aperiod of time of usage. Therefore, from an economic and practicalstandpoint, it is not only the initial selectivity of a catalyst whichis important, but also the rate at which the selectivity declines. Infact, significant improvement in lowering the decline rate of a catalystcan prove more economically attractive than a high initial selectivity.Thus, the rate at which a catalyst loses selectivity is a predominantfactor influencing the efficiency of any particular catalyst, andlowering this decline rate can lead to significant savings in terms ofminimizing waste of the ethylene starting material. As used herein,"selectivity" is used to refer to the selectivity of ethylene oxidecatalysts when measured at a given constant oxygen conversion level of40% at a gas hourly space velocity of approximately 3300 and whenmeasured after the catalyst has been placed on stream for at leastseveral days.

The catalysts of the instant invention comprise a catalyticallyeffective amount of silver, a promoting amount of alkali metal, apromoting amount of a rare earth compound, a promoting amount of rheniumand, optionally, a promoting amount of a rhenium co-promoter selectedfrom sulfur, chromium, molybdenum, tungsten and mixtures thereof,supported on a porous, refractory support.

In general, the catalysts of the present invention are prepared byimpregnating porous refractory supports with silver ions or compound(s),complex(es) and/or salt(s) dissolved in a suitable solvent sufficient tocause deposition on the support of from about 1 to about 25 percent byweight, basis the weight of the total catalyst, of silver. Theimpregnated support is then separated from the solution and thedeposited silver compound is reduced to metallic silver. Also depositedon the support either prior to, coincidentally with, or subsequent tothe deposition of the silver will be suitable ions, or compound(s)and/or salt(s) of alkali metal dissolved in a suitable solvent. Alsodeposited on the carrier coincidentally with the deposition of thesilver and/or alkali metal will be suitable rare earth compound(s),complex(es) and/or salt(s) dissolved in an appropriate solvent. Alsodeposited on the support either prior to, coincidentally with, orsubsequent to the deposition of the silver and/or alkali metal and/orrare earth will be suitable rhenium ions or compound(s), complex(es)and/or salt(s) dissolved in an appropriate solvent. In a preferredembodiment, suitable ions or salt(s), complex(es) and/or compound(s) ofsulfur, molybdenum, tungsten and/or chromium dissolved in an appropriatesolvent will be deposited on the carrier either prior to, coincidentallywith, or subsequent to the deposition of the silver and/or alkali metaland/or rare earth and/or rhenium.

The carrier or support employed in these catalysts in its broadestaspects can be any of the large number of conventional, porousrefractory catalyst carriers or support materials which are consideredrelatively inert in the presence of ethylene oxidation feeds, productsand reaction conditions. Such conventional materials are known to thoseskilled in the art and may be of natural or synthetic origin andpreferably are of a macroporous structure, i.e., a structure having asurface area below about 10 m² /g and preferably below about 3 m² /g.Particularly suitable supports are those of aluminous composition.Examples of supports which have been used as supports for differentcatalysts and which could, it is believed, be used as supports forethylene oxide catalysts are the aluminum oxides (including thematerials sold under the trade name "Alundum"), charcoal, pumice,magnesia, zirconia, keiselguhr, fuller's earth, silicon carbide, porousagglomerates comprising silica and/or silicon carbide, silica, magnesia,selected clays, artificial and natural zeolites and ceramics. Refractorysupports especially useful in the preparation of catalysts in accordancewith this invention comprise the aluminous materials, in particularthose comprising alpha alumina. In the case of alpha alumina-containingsupports, preference is given to those having a specific surface area asmeasured by the B.E.T. method of from about 0.03 m² /g to about 10 m²/g, preferably from about 0.05 m² /g to about 5 m² /g, more preferablyfrom about 0.1 m² /g to about 3 m² /g, and a water pore volume asmeasured by conventional water absorption techniques of from about 0.1to about 0.75 cc/g by volume. The B.E.T. method for determining specificsurface area is described in detail in Brunauer, S., Emmet, P. Y. andTeller, E., J. Am. Chem. Soc., 60, 309-16 (1938).

Certain types of alpha alumina containing supports are particularlypreferred. These alpha alumina supports have relatively uniform porediameters and are more fully characterized by having B.E.T. specificsurface areas of from about 0.1 m² /g to about 3 m² /g, preferably fromabout 0.1 m² /g to about 2 m2/g, and water pore volumes of from about0.10 cc/g to about 0.55 cc/g. Typical properties of some supports foundparticularly useful in the present invention are presented in Table I.Suitable manufacturers of carriers comparable to those in Table Iinclude Norton Company and United Catalysts, Inc. (UCI).

                                      TABLE I                                     __________________________________________________________________________    Carrier         A   B   C    D   E   F                                        __________________________________________________________________________    B.E.T. Surface Area, m.sup.2 /g.sup.(a)                                                       0.21                                                                              0.42                                                                              0.51 0.48                                                                              0.57                                                                              2.06                                     Water Pore Volume cc/g                                                                        0.26                                                                              0.36                                                                              0.38 0.49                                                                              0.44                                                                              0.65                                     Crush Strength, FPCS, lbs.sup.(b)                                                             100%                                                                              97% 21   14  15  No Data                                                  20  15                                                                        lbs                                                           Total Pore Volume, Hg, cc/g.sup.(c)                                                           1.26                                                                              0.42                                                                              0.40 0.46                                                                              0.42                                                                              0.65                                     Average Pore Diameter, Hg, Å.sup.(c)                                                      620 560      550 770 1000                                     Median Pore Diameter, Hg,                                                                     3.7 2.7 3.5  3.4 2.4 2.5                                      microns.sup.(c, e)                                                            Percent Pore Volume in Pores                                                                  90.0%                                                                             88.5%                                                                             93.0%                                                                              89.1%                                                                             91.5%                                                                             94.1%                                    Greater Than 350Å.sup.(c)                                                 Percent Pore Volume in Pores                                                                  87.0%                                                                             82.5%                                                                             77.0%                                                                              82.3%                                                                             83.5%                                                                             61.0%                                    Greater Than 1 Micron.sup.(c)                                                 % Wt. Alpha Alumina                                                                           99.5                                                                              98  98.8 98.5                                                                              98  70-75                                    Water Leachable Na, ppmw                                                                      12  53  69   24  18  No Data                                  Acid-Leachable Na, ppmw                                                                       40  96  188  51  45  No Data                                  Water Leachable K, ppmw                                                                       5   22  32   22  10  No Data                                  Acid-Leachable Fe, ppmw                                                                       2   5   No Data                                                                            1   5   No Data                                  % Wt. SiO.sub.2 .5  2   0.1  15  2   25-30                                    __________________________________________________________________________     .sup.(a) Method of Brunauer, Emmet and Teller, loc. cit.                      .sup.(b) Flat Plate Crush Strength, single pellet.                            .sup.(c) Determined by mercury intrusion to 55,000 psia using Micrometric     Autopore 9200 or 9210 (130° Contact angle, 0.473 N/m surface           tension of Hg).                                                               .sup.(e) Median pore diameter represents the pore diameter wherein 50% of     the total pore volume is found in pores having less than (or greater than     the median pore diameter.                                                

Of the carriers listed in TABLE I, B and C are preferred because theyprovide catalysts which have high initial selectivities.

The support, irrespective of the character of the support or carrierused, is preferably shaped into particles, chunks, pieces, pellets,rings, spheres, wagon wheels, and the like of a size suitable for use infixed bed reactors. Conventional commercial fixed bed reactors aretypically in the form of a plurality of parallel elongated tubes (in asuitable shell) approximately 0.7 to 2.7 inches O.D. and 0.5 to 2.5inches I.D. and 15-45 feet long filled with catalyst. In such reactors,it is desirable to use a support formed into a rounded shape, such as,for example, spheres, pellets, rings, tablets and the like, havingdiameters from about 0.1 inch to about 0.8 inch.

Particular supports having differing properties such as surface area andpore volume may be selected in order to provide particular catalyticproperties. With regard to surface area (B.E.T.), a possible lower limitis about 0.01 m² /g and a possible upper limit is about 10 m² /g. Withregard to water pore volume, a possible lower limit is about 0.05 cc/gand a possible upper limit is about 0.8 cc/g.

The catalysts of the present invention are prepared by a technique inwhich the alkali metal promoters, the rare earth compound, the rhenium,and the rhenium co-promoter, if present, in the form of soluble saltsand/or compounds are deposited on the catalyst and/or support prior to,simultaneously with, or subsequent to the deposition of the silver andeach other. The alkali metals may be deposited at one step of theprocess and the rare earth, rhenium and/or the rhenium co-promoter, ifpresent, at a different step or steps. The preferred method is todeposit silver, alkali metal, rare earth, rhenium and rheniumco-promoter simultaneously on the support, that is, in a singleimpregnation step, although it is believed that the individual orconcurrent deposition of the alkali metal, rare earth, rhenium andrhenium co-promoter, if present, prior to and/or subsequent to thedeposition of the silver would also produce suitable catalysts.

Promoting amounts of alkali metal or mixtures of alkali metal aredeposited on a porous support using a suitable solution. Although alkalimetals exist in a pure metallic state, they are not suitable for use inthat form. They are used as ions or compounds of alkali metals dissolvedin a suitable solvent for impregnation purposes. The carrier isimpregnated with a solution of alkali metal promoter ions, salt(s)and/or compound(s) before, during or after impregnation of the silverions or salt(s), complex(es), and/or compound(s) has taken place. Analkali metal promoter may even be deposited on the carrier afterreduction to metallic silver has taken place. The promoting amount ofalkali metal utilized will depend on several variables, such as, forexample, the surface area and pore structure and surface chemicalproperties of the carrier used, the silver content of the catalyst andthe particular ions used in conjunction with the alkali metal cation,rare earth or rhenium or rhenium co-promoter, if present, and theamounts of rare earth, rhenium and rhenium co-promoter, if any, present.The amount of alkali metal promoter deposited upon the support orpresent on the catalyst generally lies between about 10 parts permillion and about 3000 parts per million, preferably between about 15parts per million and about 2000 parts per million and more preferably,between about 20 parts per million and about 1500 parts per million byweight of total catalyst. Most preferably, the amount ranges betweenabout 50 parts per million and about 1000 parts per million by weight ofthe total catalyst. The degree of benefit obtained within theabove-defined limits will vary depending upon particular properties andcharacteristics, such as, for example, reaction conditions, catalystpreparative techniques, surface area and pore structure and surfacechemical properties of the carrier utilized, silver content of thecatalyst, and other compounds, cations or anions present in addition toalkali metal ions such as ions added with the alkali metal, rare earth,rhenium and rhenium co-promoter, if present, or compounds remaining fromthe impregnating solution, and the above-defined limits were selected tocover the widest possible variations in properties and characteristics.The effects of these variation in properties are readily determined byexperimentation. The alkali metal promoters are present on the catalystsin the form of cations (ions) or compounds of complexes or surfacecompounds or surface complexes rather than as the extremely active freealkali metals, although for convenience purposes in this specificationand claims they are referred to as "alkali metal" or "alkali metalpromoters" even though they are not present on the catalyst as metallicelements. For purposes of convenience, the amount of alkali metaldeposited on the support or present on the catalyst is expressed as themetal. Without intending to limit the scope of the invention, it isbelieved that the alkali metal compounds are oxidic compounds. Moreparticularly, it is believed that the alkali metal compounds areprobably in the form of mixed surface oxides or double surface oxides orcomplex surface oxides with the aluminum of the support and/or thesilver of the catalyst, possibly in combination with species containedin or formed from the reaction mixture, such as, for example, chloridesor carbonates or residual species from the impregnating solution(s).

In a preferred embodiment, at least a major proportion (greater than50%) of the alkali metals are selected from the group consisting oflithium, sodium, potassium, rubidium, cesium, and mixtures thereof. Asused herein, the term "alkali metal" and cognates thereof refers to thealkali metals selected from the group consisting of lithium, sodium,potassium, rubidium, cesium and mixtures thereof. As used herein, theterm "mixtures of alkali metals" or cognates of these terms refers tothe use of two or more of the alkali metals, as appropriate, to providea promoting effect. Non-limiting examples include cesium plus rubidium,cesium plus potassium, cesium plus sodium, cesium plus lithium, cesiumplus rubidium plus sodium, cesium plus potassium plus sodium, cesiumplus lithium plus sodium, cesium plus rubidium plus potassium plussodium, cesium plus rubidium plus potassium plus lithium, cesium pluspotassium plus lithium and the like. A preferred alkali metal promoteris cesium. A particularly preferred alkali metal promoter is cesium plusat least one additional alkali metal. The additional alkali metal ispreferably selected from sodium, lithium and mixtures thereof, withlithium being preferred.

It should be understood that the amounts of alkali metal promoters onthe catalysts are not necessarily the total amounts of these metalspresent in the catalyst. Rather, they are the amounts of alkali metalpromoters which have been added to the catalyst by impregnation with asuitable solution of ions, salts and/or compounds and/or complexes ofalkali metals. These amounts do not include amounts of alkali metalswhich are locked into the support, for example, by calcining, or are notextractable in a suitable solvent such as water or lower alkanol oramine or mixtures thereof and do not provide a promoting effect. It isalso understood that a source of the alkali metal promoter ions, saltsand/or compounds used to promote the catalyst may be the carrier. Thatis, the carrier may contain extractable amounts of alkali metal that canbe extracted with a suitable solvent, such as water or lower alkanol,thus preparing an impregnating solution from which the alkali metalions, salts and/or compounds are deposited or redeposited on thesupport.

As used herein, the term "compound" refers to the combination of aparticular element with one or more different elements by surface and/orchemical bonding, such as ionic and/or covalent and/or coordinatebonding. The term "ionic" or "ion" refers to an electrically chargedchemical moiety; "cationic" or "cation" being positive and "anionic" or"anion" being negative. The term "oxyanionic" or "oxyanion" refers to anegatively charged moiety containing at least one oxygen atom incombination with another element. An oxyanion is thus anoxygen-containing anion. It is understood that ions do not exist invacuo, but are found in combination with charge-balancing counterions.The term "oxidic" refers to a charged or neutral species wherein anelement in question is bound to oxygen and possibly one or moredifferent elements by surface and/or chemical bonding, such as ionicand/or covalent and/or coordinate bonding. Thus, an oxidic compound isan oxygen-containing compound which also may be a mixed, double orcomplex surface oxide. Illustrative oxidic compounds include, by way ofnon-limiting examples, oxides (containing only oxygen as the secondelement), hydroxides, nitrates, sulfates, carboxylates, carbonates,bicarbonates, oxyhalides, etc. as well as surface species wherein theelement in question is bound directly or indirectly to an oxygen eitherin the substrate or the surface.

As used herein, the term "promoting amount" of a certain component of acatalyst refers to an amount of that component which works effectivelyto provide an improvement in one or more of the catalytic properties ofthat catalyst when compared to a catalyst not containing said component.Examples of catalytic properties include, inter alia, operability(resistance to runaway), selectivity, activity, conversion, stabilityand yield. It is understood by one skilled in the art that one or moreof the individual catalytic properties may be enhanced by the "promotingamount" while other catalytic properties may or may not be enhanced andmay even be diminished. It is further understood that differentcatalytic properties may be enhanced at different operation conditions.For example, a catalyst having enhanced selectivity at one set ofoperating conditions may be operated at a different set of conditionswherein the improvement shows up in the activity rather that theselectivity and an operator of an ethylene oxide plant willintentionally change the operation conditions in order to take advantageof certain catalytic properties even at the expense of other catalyticproperties in order to maximize profits by taking into account feedstockcosts, energy costs, by-product removal costs and the like. Theparticular combination of silver, support, rare earth promoter, alkalimetal promoter, rhenium promoter and optionally, rhenium co-promoter ofthe instant invention will provide an improvement in one or morecatalytic properties over the same combination of silver, support,alkali metal promoter, rhenium promoter and optionally, rheniumco-promoter and no rare earth promoter.

As used herein, the term "catalytically effective amount of silver"refers to an amount of silver that provides a measurable conversion ofethylene and oxygen to ethylene oxide.

Promoting amounts of rare earth compounds or mixtures of rare earthcompounds are also deposited on the carrier. Although rare earth metalsexist in a pure metallic state, they are not suitable for use in thatform. They are used as ions or compounds of rare earth metals dissolvedin a suitable solvent for impregnation purposes. The carrier isimpregnated with a solution of rare earth promoter ions, salt(s) and/orcompound(s) before, during or after impregnation of the silver ions orsalt(s), complex(es), and/or compound(s) has taken place. A rare earthpromoter may even be deposited on the carrier after reduction tometallic silver has taken place, or before, during or after impregnationof the rare earth ions, salt(s) and/or compound(s) has taken place. Thepromoting amount of rare earth utilized will depend on severalvariables, such as, for example, the surface area and pore structure andsurface chemical properties of the carrier used, the silver content ofthe catalyst and the particular ions used in conjunction with the alkalimetal cation, rare earth or rhenium or rhenium co-promoter, if present,and amounts of rare earth, rhenium and rhenium co-promoter, if any,present. The amount of rare earth promoter deposited upon the support orpresent on the catalyst generally lies between about 10 parts permillion and about 1000 parts per million, and preferably between about15 parts per million and about 500 parts per million by weight of thetotal catalyst. Most preferably, the amount ranges between about 30parts per million and about 250 parts per million by weight of the totalcatalyst. The promoting effect provided by the rare earth will varydepending upon particular properties and characteristics, such as, forexample, reaction conditions, catalyst preparative techniques, surfacearea and pore structure and surface chemical properties of the carrierutilized, the silver, alkali metal, rhenium and rhenium co-promotercontent of the catalyst, and other compounds, cations or anions presentin addition to those containing the alkali metal, rare earth, rheniumand rhenium co-promoter, if present, or compounds remaining from theimpregnating solution, and the above-defined limits were selected tocover the widest possible variations in properties and characteristics.The effects of these variations in properties are readily determined byexperimentation. The rare earth promoter or promoters are presumablypresent on the catalyst in the form of oxides or oxygen-bound species,or surface compounds or surface complexes rather than as metals,although for purposes of convenience in this specification and claims,they are referred to as "rare earth(s)", "rare earth metal(s)", "rareearth metal compound (s)", "rare earth compound(s)", and/or "rare earthpromoters" even though they are not present on the catalyst as metals.For purposes of convenience, the amount of rare earth deposited on thesupport or present on the catalyst is expressed as the metal rather thanin the cationic or compounds or complexes or surface compounds orsurface complexes. Without intending to limit the scope of theinvention, it is believed that the rare earth metal compounds are oxidiccompounds. More particularly, it is believed that the rare earth metalcompounds are probably in the form of mixed surface oxides or doublesurface oxides or complex surface oxides with the aluminum of thesupport and/or the silver of the catalyst, possibly in combination withspecies contained in or formed from the reaction mixture, such as, forexample, chlorides or carbonates or residual species from theimpregnating solution(s).

As used herein, the terms "rare earth metal" and "rare earth" refer tothe rare earth metals or elements having atomic numbers 58 through 71 inthe Periodic Table of the Elements i.e., cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. As usedherein, the term "mixtures of rare earth metals" refers to the use oftwo or more of the rare earth metals having atomic numbers 58 through 71in the Periodic Table of the Elements, as appropriate, to provide apromoting effect.

In a preferred embodiment, the rare earth metal is selected from thegroup consisting of cerium, neodymium, samarium, gadolinium, dysprosium,erbium, ytterbium, and mixtures thereof, with cerium, neodymium,gadolinium, ytterbium, and mixtures thereof being particularlypreferred. Particularly preferred rare earth promoters are gadolinium,cerium and ytterbium. It should be understood that the use of Markushterminology in this specification and claims to indicate the rare earthmetals cerium and/or neodymium and/or samarium and/or gadolinium and/ordysprosium and/or erbium and/or ytterbium is not meant and does notexclude the presence, inclusion or the use of the remainder of the rareearth metals having atomic numbers 58 through 71 in the Periodic Tableof the Elements. Thus, the use of a Markush recitation in the instantspecification and claim means that the elements in the recitation areincluded, but others are not excluded, i.e., the Markush recitation isan open-ended recitation.

The promoting effect provided by the rare earth compound can be affectedby a number of variables such as for example, reaction conditions,catalyst preparative techniques, surface area and pore structure andsurface chemical properties of the support, the silver, alkali metal,rhenium and, if present, rhenium co-promoter content of the catalyst,and the presence of other cations and anions present on the catalystalone or in combination with the alkali metal and/or rare earth and/orrhenium and/or rhenium co-promoter. The presence of other activators,stabilizers, promoters, enhancers or other catalyst improvers can alsoaffect the promoting effects of the rare earth. It is understood thatany supported silver-based, alkali metal/rhenium promoted ethylene oxidecatalyst which contains other cations and/or anions or any otheractivators, promoters, enhancers, stabilizers or the catalyst improversand which contains an amount of rare earth which provides a promotingeffect, more preferably which provides higher selectivity stabilitiesthan those obtained under the same reaction conditions with the samecatalyst which contains no rare earth promoter, will fall within thescope of the instant invention and claims.

The carrier is also impregnated with rhenium ions, salt(s), compound(s),and/or complex(es). This may be done at the same time that the alkalimetal promoter is added, or before or later; or at the same time therare earth promoter is added, or before or later; or at the same timethat the silver is added, or before or later; or at the same time thatthe rhenium co-promoter, if present, is added, or before or later.Preferably, rhenium, rare earth, alkali metal, rhenium co-promoter, ifpresent, and silver are in the same impregnating solution, although itis believed that their presence in different solutions will stillprovide suitable catalysts. The preferred amount of rhenium, calculatedas the metal, deposited on or present on the carrier or catalyst rangesfrom about 0.1 micromoles per gram to about 10 micromoles per gram, morepreferably from about 0.2 micromoles per gram to about 5 micromoles pergram of total catalyst, or, alternatively stated, from about 19 partsper million to about 1860 parts per million, preferably from about 37parts per million to about 930 parts per million by weight of totalcatalyst. The degree of benefit obtained within the above-defined limitswill vary depending upon particular properties and characteristics, suchas, for example, reaction conditions, catalyst preparative conditions,surface area and pore structure and surface chemical properties of thecarrier utilized, silver content of the catalyst and other compounds,anions or cations present in addition to those containing rhenium,alkali metal, rare earth, or rhenium co-promoter, such as ions addedwith the alkali metal, rare earth, rhenium or rhenium co-promoter, orcompounds remaining from the impregnating technique, and theabove-defined limits were selected to cover the widest possiblevariations in properties and characteristics. The effects of thesevariations are readily determined by experimentation. For purposes ofconvenience, the amount of rhenium present on the catalyst is expressedas the metal, irrespective of the form in which it is present.

The rhenium compounds used in the preparation of the instant catalystsare rhenium compounds that can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver and thealkali metal promoter. Examples of suitable rhenium compounds includethe rhenium salts such as rhenium halides, the rhenium oxyhalides, therhenates, the perrhenates, the oxides and the acids of rhenium. Apreferred compound for use in the impregnation solution is theperrhenate, preferably ammonium perrhenate. However, the alkali metalperrhenates, alkaline earth metal perrhenates, silver perrhenates, otherperrhenates and rhenium heptoxide can also be suitably utilized. Rheniumheptoxide, Re₂ O₇, when dissolved in water, hydrolyzes to perrhenicacid, HReO₄, or hydrogen perrhenate. Thus, for purposes of thisspecification, rhenium heptoxide can be considered to be a perrhenate,i.e., ReO₄. It is also understood that there are many rhenium compoundsthat are not soluble per se in water. However, these compounds can besolubilized by utilizing various acids, bases, peroxides, alcohols, andthe like. After solubilization, these compounds could be used, forexample, with an appropriate amount of water or other suitable solventto impregnate the carriers. Of course, it is also understood that uponsolubilization of many of these compounds, the original compound nolonger exists after solubilization. For example, rhenium metal is notsoluble in water. However, it is soluble in concentrated nitric acid aswell as in hydrogen peroxide solution. Thus, by using an appropriatereactive solvent, one could use rhenium metal to prepare a solubilizedrhenium-containing impregnating solution. In a preferred embodiment ofthe instant invention, the rhenium present on the catalyst is present ina form that is extractable in a dilute aqueous base solution.

It was found in U.S. Pat. No. 4,766,105, issued Aug.23, 1988, that if arhenium co-promoter is added to an alkali metal/rhenium doped supportedsilver catalyst, an improvement in initial selectivity is obtained.While suitable catalysts can be prepared in the absence of a rheniumco-promoter, it is preferable that the catalyst in the present inventioncontain a rhenium co-promoter. When a co-promoter is utilized, theco-promoter is a selected from the group consisting of sulfur,molybdenum, tungsten, chromium and mixtures thereof, preferably acompound of sulfur, molybdenum, tungsten, chromium and mixtures thereof.The exact form of the co-promoter on the catalyst is not known, Theco-promoter, it is believed, is not present on the catalyst in theelemental form since the co-promoter is applied to catalyst in the formof ions, salts, compounds and/or complexes and the reducing conditionsgenerally used to reduce the silver to metallic silver are not usuallysufficient to reduce the sulfur, molybdenum, tungsten or chromium to theelemental form. It is believed that the co-promoter deposited on thesupport or present on the catalyst is in the compound form, and probablyin the form of an oxygen-containing or oxidic compound. In a presentlypreferred embodiment, the co-promoter is applied to the catalyst in theoxyanionic form, i.e, in the form of an anion, or negative ion whichcontains oxygen. Examples of anions of sulfur that can be suitablyapplied include sulfate, sulfite, bisulfite, bisulfate, sulfonate,persulfate, thiosulfate, dithionate, etc. Preferred compounds to beapplied are ammonium sulfate and the alkali metal sulfates. Examples ofanions of molybdenum, tungsten and chromium that can be suitably appliedinclude molybdate, dimolybdate, paramolybdate, other iso- andhetero-polymolybdates, etc.; tungstate, paratungstate, metatungstate,other iso- and hetero-polytungstates, etc.; and chromate, dichromate,chromite, halochromate, etc. Preferred are sulfates, molybdates,tungstates and chromates. The anions can be supplied with variouscounter-ions. Preferred are ammonium, alkali metal and hydrogen (i.e.acid form). The anions can be prepared by the reactive dissolution ofvarious non-anionic materials such as the oxides such as SO₂, SO₃, MoO₃,WO₃, Cr₂ O₃, etc., as well as other materials such as halides,oxyhalides, hydroxyhalides, hydroxides, sulfides, etc., of the metals.

When a co-promoter is used, the carrier is impregnated with rheniumco-promoter ions, salt(s), compound(s) and/or complex(es). This may bedone at the same time that the other components are added, or beforeand/or later. Preferably, rhenium co-promoter, rhenium, rare earth,alkali metal and silver are in the same impregnating solution, althoughit is believed that their presence in different solutions will stillprovide suitable catalysts.

The preferred amount of co-promoter compound present on or deposited onthe support or catalyst ranges from about 0.1 to about 10 micromoles,preferably from about 0.2 to about 5 micromoles, expressed as theelement, per gram of total catalyst. The degree of benefit obtainedwithin the above-defined limits will vary depending upon particularproperties and characteristics, such as, for example, reactionconditions, catalyst preparative techniques, surface area and porestructure and surface chemical properties of the carrier utilized,silver content of the catalyst, alkali content of the catalyst, rheniumcontent of the catalyst, and other compounds anions or cations presentbesides those containing rhenium, rhenium co-promoter and alkali metal,such as ions added with the alkali metal, rhenium or rheniumco-promoter, or compounds remaining from the impregnation technique, andthe above-defined limits were selected to cover the widest possiblevariations in properties and characteristics. These variations arereadily determined by experimentation. For purposes of convenience theamount of co-promoter present on the catalyst is expressed as theelement irrespective of the form in which it is present.

The presence of the indicated and claimed promoting amount of rheniumco-promoters in this specification and claims does not preclude the useof other activators, promoters, enhancers, stabilizers, improvers, etc.,and it is not intended by the use of Markush terminology in thisspecification and claims to exclude the use of such other activators,promoters, enhancers, stabilizers, improvers, etc.

The co-promoter compounds, salts and/or complexes suitable for use inthe preparation of the instant catalysts are compounds, salts and/orcomplexes which can be solubilized in an appropriate solvent.Preferably, the solvent is a water-containing solvent. More preferably,the solvent is the same solvent used to deposit the silver, alkali metalpromoter and rhenium. Preferred co-promoter compounds are the oxyanioniccompounds of the co-promoter elements, preferably the ammonium andalkali metal oxyanionates, such as ammonium sulfate, potassium sulfate,cesium chromate, rubidium tungstate, ammonium molybdate, lithiumsulfate, sodium tungstate, lithium chromate and the like.

Generally, the carrier is contacted with a silver salt, a silvercompound, or a silver complex which has been dissolved in an aqueoussolution, so that the carrier is impregnated with said aqueous solution;thereafter the impregnated carrier is separated form the aqueoussolution, e.g., by centrifugation or filtration and then dried. The thusobtained impregnated carrier is heated to reduce the silver to metallicsilver. It is conveniently heated to a temperature in the range of fromabout 50° C. to about 600° C., during a period sufficient to causereduction of the silver salt, compound or complex to metallic silver andto form a layer of finely divided silver, which is bound to the surfaceof the carrier, both the exterior and pore surface. Air, or otheroxidizing gas, reducing gas, an inert gas or mixtures thereof may beconducted over the carrier during this heating step.

There are several known methods to add the silver to the carrier orsupport. The carrier may be impregnated with an aqueous solutioncontaining silver nitrate dissolved therein, and then dried, after whichdrying step the silver nitrate is reduced with hydrogen or hydrazine.The carrier may also be impregnated with an ammoniacal solution ofsilver oxalate or silver carbonate, and then dried, after which dryingstep the silver oxalate or silver carbonate is reduced to metallicsilver by heating, e.g., to about 600° C. Specific solutions of silversalts with solubilizing and reducing agents may be employed as well,e.g., combinations of the vicinal alkanolamines, alkyldiamines andammonia. One such example of a solution of silver salts comprises animpregnating solution comprising a silver salt of a carboxylic acid, anorganic amine alkaline solubilizing/reducing agent, and an aqueoussolvent.

Suitable carboxylic acid silver salts include silver carbonate and thesilver salts of mono- and polybasic carboxylic and hydrocarboxylic acidsof up to about 16 carbon atoms. Silver carbonate and silver oxalate areparticularly useful silver salts, with silver oxalate being mostpreferred.

An organic amine solubilizing/reducing agent is present in theimpregnating solution. Suitable organic aminesilver-solubilizing/reducing agents include lower alkylenediamines offrom 1 to 5 carbon atoms, mixtures of a lower alkanolamine of from 1 to5 carbon atoms with a lower alkylenediamine of from 1 to 5 carbon atoms,as well as mixtures of ammonia with lower alkanolamines or loweralkylenediamines for from 1 to 5 carbons. Four groups of organic aminesolubilizing/reducing agents are particularly useful. The four groupsinclude vicinal alkylenediamines of from 2 to 4 carbon atoms, mixturesof (1) vicinal alkanolamines of from 2 to 4 carbon atoms and (2) vicinalalkylenediamines of from 2 to 4 carbon atoms, mixtures of vicinalalkylenediamines of from 2 to 4 carbon atoms and ammonia, and mixturesof vicinal alkanolamines of from 2 to 4 carbon atoms and ammonia. Thesesolubilizing/reducing agents are generally added in the amount of fromabout 0.1 to about 10 moles per mole of silver present.

Particularly preferred solubilizing/reducing agents are ethylenediamine,ethylenediamine in combination with ethanolamine, ethylenediamine incombination with ammonia, and ethanolamine in combination with ammonia,with ethylenediamine being most preferred. Ethylenediamine incombination with ethanolamine gives comparable results, but it isbelieved that impurities present in certain commercially availableethanolamine preparations can produce inconsistent results.

When ethylenediamine is used as the sole solubilizing/reducing agent, itis necessary to add amount of the amine in the range of from about 0.1to about 5.0 moles of ethylenediamine per mole of silver.

When ethylenediamine and ethanolamine together are used as thesolubilizing/reducing agent, it is suitable to employ from about 0.1 toabout 3.0 moles of ethylenediamine per mole of silver and from about 0.1to about 2.0 moles of ethanolamine per mole of silver.

When ethylenediamine or ethanolamine is used with ammonia, it isgenerally useful to add at least about two moles of ammonia per mole ofsilver and very suitable to add from about 2 to about 10 moles ofammonia per mole of silver. The amount of ethylenediamine orethanolamine employed then is suitably from 0.1 to 2.0 moles per mole ofsilver.

One method of preparing the silver containing catalyst can be found inU.S. Pat. No. 3,702,259, issued Nov. 7, 1972, incorporated by referenceherein. Other methods for preparing the silver-containing catalystswhich in addition contain higher alkali metal promoters can be found inU.S. Pat. No. 4,010,115, issued Mar. 1, 1977; and U.S. Pat. No.4,356,312, issued Oct. 26, 1982; U.S. Pat. No. 3,962,136, issued Jun. 8,1976 and U.S. Pat. No. 4,012,425, issued Mar. 15, 1977, all incorporatedby reference herein. Methods for preparing silver-containing catalystscontaining higher alkali metal and rhenium promoters can be found inU.S. Pat. No. 4,761,394, issued Aug. 2, 1988, which is incorporated byreference herein, and methods for silver-containing catalysts containinghigher alkali metal and rhenium promoters and a rhenium co-promoters canbe found in U.S. Pat. No. 4,766,105, issued Aug. 2, 1988, which isincorporated herein be reference.

The preferred amount of alkali metal promoter deposited on or present onthe surface of the carrier of catalyst generally lies between about 10parts per million and about 3000 parts per million, preferably betweenabout 15 parts per million and about 2000 parts per million and morepreferably between about 20 parts per million and about 1500 parts permillion by weight of alkali metal calculated on the total carriermaterial. Amounts between about 50 parts per million and about 1000parts per million are most preferable. Suitable compounds of alkalimetals comprise lithium, sodium, potassium, rubidium, cesium or mixturesthereof in a promoting amount with the even more preferred promotersbeing rubidium and/or cesium plus an additional alkali metal selectedfrom lithium, sodium and mixtures thereof. Preferably the amount rangesfrom about 10 parts per million and about 3000 parts per million, morepreferably between about 15 parts per million and about 2000 parts permillion, even more preferably between about 20 parts per million andabout 1500 parts per million by weight, and most preferably betweenabout 50 parts per million and 1000 parts per million by weight. Themost preferred promoter is cesium plus lithium, preferably applied in anaqueous solution having cesium nitrate or cesium hydroxide dissolvedtherein.

There are known excellent methods of applying the promoterscoincidentally with the silver on the carrier. Suitable alkali metalsalts are generally those which are soluble in the silver-impregnatingliquid phase. Besides the above-mentioned compounds may be mentioned thenitrites; the halides, such as fluorides, chlorides, iodides, bromides;oxyhalides; bicarbonates; borates; sulfates; sulfites; bisulfates;acetates; tartrates; lactates and isopropoxides, etc. The use of alkalimetal, rare earth, rhenium or co-promoter salts which have ions whichreact with the silver salt in solution is preferably avoided, e.g. theuse of cesium chloride together with silver nitrate in an aqueoussolution, since then some silver chloride is prematurely precipitated.Here the use of cesium nitrate is recommended instead of cesiumchloride, for example. However, cesium chloride may be used togetherwith a silver salt-amine-complex in aqueous solution, since then thesilver chloride is not precipitated prematurely from the solution.

The promoters may be deposited on the carrier (support) or on thecatalyst, depending upon the particular impregnation technique orsequence utilized. In this specification and claims, the term "on thecatalyst" when referring to the deposition or presence of promotersand/or co-promoters refers to the catalyst which comprises thecombination of carrier (support) and silver. Thus, the promoters, i.e.,alkali metal, rare earth, rhenium and rhenium co-promoter may be foundindividually or in a mixture thereof on the catalyst, on the support oron both the catalyst and the support. There may be, for example, alkali,rare earth, rhenium and rhenium co-promoter on the support; alkalimetal, rare earth, rhenium and rhenium co-promoter on the catalyst;alkali metal, rare earth, and rhenium on the support and rheniumco-promoter on the catalyst; alkali metal, rare earth, and rheniumco-promoter on the support and rhenium on the catalyst; alkali metal,rare earth, rhenium and rhenium co-promoter on the support and rheniumand rhenium co-promoter on the catalyst and any of the other possibledistributions of alkali metal, rare earth, rhenium and/or rheniumco-promoter between the support and/or the catalyst.

The amount of the alkali metal and/or rare earth and/or rheniumpromoters and/or rhenium co-promoters on the porous carrier or catalystmay also be regulated within certain limits by washing out the surplusof promoter material with an appropriate solvent, for example, methanolor ethanol.

A particularly preferred process of impregnating the carrier consists ofimpregnating the carrier with an aqueous solution containing a silversalt of a carboxylic acid, an organic amine, a salt of cesium, ammoniumperrhenate and ammonium sulfate dissolved therein. Silver oxalate is apreferred salt. It can be prepared by reacting silver oxide (slurry inwater) with (a) a mixture of ethylenediamine and oxalic acid, or (b)oxalic acid and then ethylenediamine, which latter is preferred, so thatan aqueous solution of silver oxalate-ethylenediamine complex isobtained, to which solution is added a certain amount of cesiumcompound, ammonium perrhenate and ammonium sulfate. While addition ofthe amine to the silver oxide before adding the oxalic acid is possible,it is not preferred since it can give rise to solutions which areunstable or even explosive in nature. Other diamines and other amines,such as ethanolamine, may be added as well. A cesium-containing silveroxalate solution may also be prepared by precipitating silver oxalatefrom a solution of cesium oxalate and silver nitrate and rinsing withwater or alcohol the obtained silver oxalate in order to remove theadhering cesium salt until the desired cesium content is obtained. Thecesium-containing silver oxalate is then solubilized with ammonia and/oran amine in water and ammonium perrhenate and ammonium sulfate is added.Rubidium-, potassium-, sodium-, lithium- and mixtures of alkalimetal-containing solutions may be prepared also in these ways. Theimpregnated carriers are then heated to a temperature between about 50°C. and about 600° C., preferably between about 75° C. and about 400° C.to evaporate the liquid and produce a metallic silver.

In general terms, the impregnation process comprises impregnating thesupport with one or more solutions-comprising silver, alkali metal, rareearth, rhenium and rhenium co-promoter. As used in the instantspecification and claims, the terminology "impregnating the support withone or more solutions comprising silver, alkali metal, rare earth,rhenium and/or rhenium co-promoter" and similar or cognate terminologymeans that the support is impregnated in a single or multipleimpregnation with one solution containing silver, alkali metal, rareearth, rhenium and rhenium co-promoter in differing amounts; or inmultiple impregnations with two or more solutions, wherein each solutioncontains at least one component selected from silver, alkali metal, rareearth, rhenium and rhenium co-promoter, with the proviso that all of thecomponents of silver, alkali metal, rare earth, rhenium and rheniumco-promoter will individually be found in at least one of the solutions.The concentration of the silver (expressed as the metal) in thesilver-containing solution will range from about 1g/l up to thesolubility limit when a single impregnation is utilized. Theconcentration of the alkali metal (expressed as the metal) will rangefrom about 1×10⁻³ g/l up to about 12 g/l and preferably, from about10×10⁻³ g/l to about 12 g/l when a single impregnation step is utilized.The concentration of the rare earth (expressed as the element) willrange from about 1×10⁻⁴ g/l up to about 1 g/l and preferably, from about5×10⁻⁴ g/l to about 0.1 g/l when a single impregnation step is utilized.The concentration of the rhenium (expressed as the metal) will rangefrom about 5×10⁻³ g/l to about 20 g/l and preferably from about 50×10⁻³g/l to about 20 g/l when a single impregnation step is utilized. Theconcentration of rhenium co-promoter (expressed as the element) willrange from about 1×10⁻³ g/l to about 20 g/l and preferably from about10×10⁻³ g/l to about 20 g/l when a single impregnation step is utilized.Concentrations selected within the above noted ranges will depend uponthe pore volume of the catalyst, the final amount desired in the finalcatalyst and whether the impregnation is single or multiple. Appropriateconcentrations can be readily determined by routine experimentation.

The amount of silver deposited on the support or present on the supportis to be a catalytically effective amount of silver, i.e., an amountthat catalyzes the reaction of ethylene and oxygen to produce ethyleneoxide. Preferably this amount will range from about 1 to about 30percent by weight of the total catalyst, more preferably from about 1 toabout 25 percent by weight of the total catalyst, and even morepreferably from about 5 to about 20 percent by weight of the totalcatalyst. The upper and lower limits of preferred silver concentrationscan be suitably varied, depending upon the particular catalyticproperties or effect desired or the other variables involved. The lowerlimit of silver is probably about 1 percent by weight of the totalcatalyst, and the upper limit of silver is probably about 30 percent byweight of the total catalyst.

The amount of alkali metal deposited on the support or catalyst orpresent on the support or catalyst is to be a promoting amount.Preferably the amount will range from about 10 parts per million toabout 3000 parts per million, more preferably from about 15 parts permillion to about 2000 parts per million and even more preferably fromabout 20 parts per million to about 1500 parts per million, and mostpreferably, from about 50 parts per million to about 1000 parts permillion by weight of the total catalyst, expressed as the metal. Theupper and lower limits of preferred alkali metal concentrations can besuitably varied depending upon the particular promoting effect desiredor other variables involved. A possible lower limit of alkali metal is,for example, about 1 parts per million by weight of the total catalyst,expressed as the metal, and a possible upper limit of alkali metal is,for example, about 3000 parts per million by weight of the totalcatalyst, expressed as the metal.

The amount of rare earth deposited on the support or catalyst or presenton the support or catalyst is to be a promoting amount. Preferably, theamount will range from about 10 parts per million to about 1000 partsper million, preferably from about 15 parts per million to about 500parts per million, and more preferably, from about 30 parts per millionto about 250 parts per million by weight of the total catalyst,expressed as the rare earth element. The upper and lower limits ofpreferred rare earth concentrations can be suitably varied dependingupon the particular promoting effect desired or other variablesinvolved. A possible lower limit of rare earth is, for example, about 1part per million by weight of the total catalyst, expressed as the rareearth element, and a possible upper limit of rare earth is, for example,about 2,000 parts per million by weight of the total catalyst, expressedas the rare earth element.

The amount of rhenium deposited on the support or catalyst or present onthe support of catalyst is to be a promoting amount. Preferably, theamount will range from about 0.1 micromoles per gram to about 10micromoles per gram, preferably from about 0.2 micromoles per gram toabout 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the metal. The upper and lower limits of preferred rheniumconcentrations can be suitably varied depending upon the particularpromoting effect desired or other variables involved. A possible lowerlimit of rhenium is, for example, about 0.01 micromoles per gram oftotal catalyst, and a possible upper limit of rhenium is, for example,about 16 micromoles per gram of total catalyst, expressed as the metal.

The amount of rhenium co-promoter deposited on the support or catalystor present on the support or catalyst is to be a promoting amount.Preferably, the amount will range from about 0.1 micromoles per gram toabout 10 micromoles per gram, preferably from about 0.2 micromoles pergram to about 5 micromoles per gram, and more preferably, from about 0.5micromoles per gram to about 4 micromoles per gram of total catalyst,expressed as the element or the metal. The upper and lower limits ofpreferred rhenium co-promoter concentrations can be suitably varieddepending upon the particular promoting effect desired or othervariables involved. A possible lower limit of rhenium co-promoter is,for example, about 0.01 micromoles/gram of total catalyst, and apossible upper limit of rhenium co-promoter is, for example, about 16micromoles/gram of total catalyst, measured as the metal.

It is observed that independent of the form in which the silver ispresent in the solution before precipitation on the carrier, the term"reduction to metallic silver" is used, while in the meantime oftendecomposition by heating occurs. We prefer to use the term "reduction",since Ag⁺ ion is converted into a metallic Ag atom. Reduction times maygenerally vary from about 0.5 minute to about 8 hours, depending on thecircumstances.

The silver catalysts according to the present invention have been shownto have a particularly high selectivity stability for ethylene oxideproduction in the direct oxidation of ethylene with molecular oxygen toethylene oxide. The conditions for carrying out such an oxidationreaction in the presence of the silver catalysts according to thepresent invention broadly comprise those already described in the priorart. This applies, for example, to suitable temperatures, pressures,residence times, diluent materials such as nitrogen, carbon dioxide,steam, argon, methane or other saturated hydrocarbons, to the presenceof moderating agents to control the catalytic action, for example,1,2-dichloroethane, vinyl chloride, ethyl chloride or chlorinatedpolyphenyl compounds, to the desirability of employing recycleoperations or applying successive conversations in different reactors toincrease the yields of ethylene oxide, and to any other specialconditions which may be selected in processes for preparing ethyleneoxide. Pressures in the range of from atmospheric to about 500 psig aregenerally employed. Higher pressures, however, are not excluded.Molecular oxygen employed as reactant can be obtained from conventionalsources. The suitable oxygen charge may consist essentially orrelatively pure oxygen, a concentrated oxygen stream comprising oxygenin major amount with lesser amounts of one or more diluents, such asnitrogen and argon, or another oxygen-containing stream, such as air. Itis therefore evident that the use of the present silver catalysts inethylene oxide reactions is in no way limited to the use of specificconditions among those which are known to be effective. For purposes ofillustration only, the following table shows the range of conditionsthat are often used in current commercial ethylene oxide reactor units.

                  TABLE II                                                        ______________________________________                                        *GHSV                   1500-10,000                                           Inlet Pressure          150-400 psig                                          Inlet Feed                                                                    Ethylene                1-40%                                                 O.sub.2                 3-12%                                                 Ethane                  0-3%                                                  Argon and/or methane and/or nitrogen                                                                  0.3-20 ppmv total                                     diluent chlorohydrocarbon moderator                                           Coolant temperature     180-315° C.                                    Catalyst temperature    180-325° C.                                    O.sub.2 conversion level                                                                              10-60%                                                EO Production (Work Rate)                                                                             2-16 lbs. EO/cu.                                                              ft. catalyst/hr.                                      ______________________________________                                         *Cubic feet of gas at standard temperature and pressure passing over one      cubic foot of packed catalyst per hour.                                  

In a preferred application of the silver catalysts according to thepresent invention, ethylene oxide is produced when an oxygen-containinggas is contacted with ethylene in the presence of the present catalystsat a temperature in the range of from about 180° C. to about 330° C. andpreferably about 200° C. to about 325° C.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The invention will be illustrated by the following illustrativeembodiments which are provided for illustration only and are notintended to limit the scope of the instant invention.

ILLUSTRATIVE EMBODIMENTS Illustrative Embodiment 1

The following illustrative embodiment describes typical preparativetechniques for making the catalysts of the instant invention (andcomparative catalysts) and the typical technique for measuring theproperties of these catalysts.

Part A: Preparation of stock silver oxalate/ethylene-diamine solutionfor use in catalyst preparation:

1) Dissolve 415 grams (g) of reagent-grade sodium hydroxide in 2340milliliters (ml) deionized water. Adjust the temperature to 50° C.

2) Dissolve 1699 g of "Spectropure" (high purity) silver nitrate in 2100ml deionized water. Adjust the temperature to 50° C.

3) Add sodium hydroxide solution slowly to silver nitrate solution withstirring while maintaining a temperature of 50° C. Stir for 15 minutesafter addition is complete, and then lower the temperature to 40° C.

4) Insert clean filter wands and withdraw as much water as possible fromthe precipitate created in step (3) in order to remove sodium andnitrate ions. Measure the conductivity of the water removed and add backas much fresh deionized water as was removed by the filter wands. Stirfor 15 minutes at 40° C. Repeat this process until the conductivity ofthe water removed is less than 90 μmho/cm. Then add back 1500 mldeionized water.

5) Add 630 g of high-purity oxalic acid dihydrate in approximately 100gincrements. Keep the temperature at 40° C. and stir to mix thoroughly.Add the last portion of oxalic acid dihydrate slowly and monitor pH toensure that pH does not drop below 7.8.

6) Remove as much water from the mixture as possible using clean filterwands in order to form a highly concentrated silver-containing slurry.Cool the silver oxalate slurry to 30° C.

7) Add 699 g of 92 percent weight (% w) ethylenediamine (8% deionizedwater). Do not allow the temperature to exceed 30° C. during addition.

The above procedure yields a solution containing approximately 27-33% wsilver.

Part B: Preparation of impregnation solutions

For Catalyst A (Gd/Re), into a 10 ml beaker is added 0.166 g of(NH₄)ReO₄ and approximately 2.0 g of ethylene-diamine/H₂ O (50/50 byweight), and the mixture is allowed to dissolve with stirring. 0.079 gof Li₂ SO₄.H₂ O is dissolved in 1 ml of water in a weighing dish, andthen added to the perrhenate solution. 0.3416 g of LiNO₃ is dissolved in2 ml of water and added to the perrhenate solution. Theperrhenate/lithium sulfate/lithium nitrate solution is allowed to stir,ensuring complete dissolution. Separately, 0.0839 g of Gd(OAc)₃.4H₂ O isdissolved in 3 ml of water. Both dopant solutions are then added to 184g of the above-prepared silver solution (specific gravity=1.556 g/cc),and the resulting solution is diluted with water to a total weight of204 g. One-fourth of this solution is used to prepare a catalyst. 0.0475g of CsOH is added to a 51 g portion of the silver oxalate/dopantsolution to prepare the final impregnation solution.

For Catalyst B (Ce/Re), the procedure for Catalyst A is followed, exceptthat 0.656g of Ce(OAc)₃ is used in place of Gd(OAc)₃.4H₂ O.

For Catalyst C (Yb/Re), the procedure for Catalyst A is followed, exceptthat 0.0873g of Yb(OAc)₃ is used in place of Gd(OAc)₃.4H₂ O.

For Catalyst D (Re only), the procedure for Catalyst A is followed,except that no Gd(OAc)₃.4H₂ O is added.

Part C: Catalyst impregnation and curing

Catalyst carrier C which is described in Table 1 is a preferred supportfor the instant invention and is used in the following examples andillustrative embodiments unless otherwise stated.

Approximately 30 g of carrier C are placed under 25mm vacuum for 3minutes at room temperature. Approximately 50 g of doped impregnatingsolution is then introduced to submerge the carrier, and the vacuum ismaintained at 25 mm for an additional 3 minutes. At the end of thistime, the vacuum is released, and excess impregnating solution isremoved from the carrier by centrifugation for 2 minutes at 500 rpm. Ifthe impregnating solution is prepared without monoethanolamine, then theimpregnated carrier is then cured by being continuously shaken in a 300cu. ft/hr. air stream flowing across a cross-sectional area ofapproximately 3-5 square inches at 250°-270° C. for 5-6 minutes. Ifsignificant monoethanolamine is present in the impregnating solution,then the impregnated carrier is cured by being continuously shaken in a300 cu. ft./hr. air stream at 250° C. for 2.5 minutes, followed by a 100cu. ft./hr. air stream at 270° C. for 7.5 minutes (all over across-section area of approximately 3-5 square inches). The curedcatalyst is then ready for testing.

This procedure will yield catalysts on this carrier which containapproximately. 13.5% w Ag with the following approximate dopant levels(expressed in parts per million by weight basis the weight of the totalcatalyst, i.e., ppmw) and which are approximately optimum in cesium forthe given silver and rhenium and sulfur levels and support with regardto initial selectivity under the test conditions described below.

    ______________________________________                                                      Rare Earth,                                                     Cs, ppmw      ppmw       Re, ppmw  S, ppmw                                    ______________________________________                                        Catalyst A                                                                            460       79         280     48                                       Catalyst B                                                                            430       70         280     48                                       Catalyst C                                                                            460       86         280     48                                       Catalyst D                                                                            430       None       280     48                                       ______________________________________                                    

The actual silver content of the catalyst can be determined by any of anumber of standard, published procedures. The actual level of rhenium onthe catalysts prepared by the above process can be determined byextraction with 20 mM aqueous sodium hydroxide, followed byspectrophotometric determination of the rhenium in the extract. Theactual level of rare earth on the catalyst can be determined by standardatomic emission spectroscopy. The actual level of cesium on the catalystcan be determined by employing a stock cesium hydroxide solution, whichhas been labeled with a radioactive isotope of cesium, in catalystpreparation. The cesium content of the catalyst can then be determinedby measuring the radioactivity of the catalyst. Alternatively, thecesium content of the catalyst can be determined by leaching thecatalyst with boiling deionized water. In this extraction processcesium, as well as other alkali metals, is measured by extraction fromthe catalyst by boiling 10 grams of whole catalyst in 20 milliliters ofwater for 5 minutes, repeating the above two more times, combining theabove extractions and determining the amount of alkali metal present bycomparison to standard solutions of reference alkali metals using atomicabsorption spectroscopy (using Varian Techtron Model 1200 orequivalent). It should be noted that the cesium content of the catalystas determined by the water leaching technique may be lower than thecesium content of the catalyst as determined by the radiotracertechnique.

Part D: Standard Microreactor Catalyst Test Conditions/Procedure

3 to 5 grams of crushed catalyst (14-20 mesh) are loaded into a 1/4 inchdiameter stainless steel U-shaped tube. The U tube is immersed in amolten metal bath (heat medium) and the ends are connected to a gas flowsystem. The weight of the catalyst used and the inlet gas flow rate areadjusted to achieve a gas hourly space velocity of 3300 cc of gas per ccof catalyst per hour. The inlet gas pressure is 210 psig.

The gas mixture passed thorough the catalyst bed (in once-throughoperation) during the entire test run (including startup) consists of30% ethylene, 8.5% oxygen, 5% carbon dioxide, 54.5% nitrogen, and 2.0 to6.0 ppmv ethyl chloride.

The initial reactor (heat medium) temperature is 225° C. After 1 hour atthis initial temperature, the temperature is increased to 235° C. for 1hour, and then adjusted to 245° C. for 1 hour. The temperature is thenadjusted so as to achieve a constant oxygen conversion level of 40%.Performance data at this conversion level are usually obtained when thecatalyst has been onstream for a total of at least 2-3 days. Due toslight differences in feed gas composition, gas flow rates, and thecalibration of analytical instruments used to determine the feed andproduct gas compositions, the measured selectivity and activity of agiven catalyst may vary slightly from one test run to the next. To allowmeaningful comparison of the performance of catalysts tested atdifferent times, all catalysts described in this illustrative embodimentwere tested simultaneously with a reference catalyst. All performancedata reported in this illustrative embodiment are corrected relative tothe average initial performance of the reference catalyst (S₄₀ =81.0%;T₄₀ =230° C).

After obtaining initial performance values for selectivity at 40%conversion the catalysts are subjected to high severity agingconditions. Under these conditions, the catalyst is brought to either85% conversion or a maximum of 285° C. for a 10-day period to acceleratethe aging of the catalyst. After this 10-day aging period, the catalystsare again brought to 40% conversion and reoptimized under standardconditions. The selectivity is again measured, and compared to theoriginal value of the fresh catalyst. After the new selectivity value isdetermined, this cycle is repeated, and the selectivity decline of thecatalyst is continuously measured under standard 40% conversionconditions in 10-day cycles relative to its fresh initial performance.The results are presented below in Table III. All selectivity values areexpressed as %. The initial performances of Catalysts A, B, C and D weredetermined to be the same, within experimental error. Initial S₄₀ valuesof 86.0% ±0.4% and T₄₀ values of 260° C. ±3° C. were obtained.

Loss is Selectivity (%)={S₄₀, % (Aged)}-{S₄₀, % (Fresh)}

                  TABLE III                                                       ______________________________________                                        Loss of Selectivity from Fresh Catalyst                                       Total Days at Either 85% Conversion or 285° C.                         (Data obtained at 40% Conversion Conditions)                                  Catalyst                                                                              10 Days  20 Days  30 Days                                                                              40 Days                                                                              50 Days                               ______________________________________                                        A (Re/Gd)                                                                             0%       -0.8%    -1.4%  -2.2%  -3.4%                                 B (Re/Ce)                                                                             0%          0%    -0.4%  --     --                                    C (Re/Yb)                                                                             0%       --       --     --     --                                    D (Re)  -1.2%    -2.3%    -3.4%  -5.0%  -6.1%                                 ______________________________________                                    

As mentioned previously, selectivity decline is of tremendous economicimportance when choosing a catalyst, and retarding this decline rate canlead to significant savings in costs. As can be seen from Table III,Catalyst D, which does not contain a rare earth element in combinationwith Re, decreases in selectivity much more rapidly than do thecatalysts which contain a rare earth element. Catalysts which containrare earth elements in combination with rhenium maintain theirselectivity significantly longer than catalysts without added rare earthelements and are thus significantly advantaged.

What is claimed is:
 1. In a process for the production of ethylene oxidewherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising a catalytically effective amountof silver, a promoting amount of alkali metal, a promoting amount of arare earth metal compound and a promoting amount of rhenium supported ona suitable support having a surface area in the range of from about 0.05m² /g to about 10 m² /g.
 2. In a process for the production of ethyleneoxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising silver, an alkali metal promoter,from about 10 parts per million to about 1000 parts per million of rareearth promoter, expressed as the element, and rhenium promoter supportedon a porous refractory support having a surface area in the range offrom about 0.05 m² /g to about 10 m² /g, wherein the rare earth isapplied to the support in the form of an oxide or an oxygen-containingbound species.
 3. In a process for the production of ethylene oxidewherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising from about 1 percent by weight toabout 25 percent by weight of the total catalyst of the silvercompound(s), measured as the metal, from about 10 parts per million toabout 3000 parts per million by weight of alkali metal compound(s),expressed as the metal, per million parts by weight of the totalcatalyst, from about 10 parts per million to about 1000 parts permillion by weight of the total catalyst of rare earth metal compound(s),expressed as the element, and from about 0.1 micromoles to about 10micromoles per gram of total catalyst of rhenium compound(s), expressedas the metal, to provide the catalyst with a catalytically effectiveamount of silver, a promoting amount of alkali metal, a promoting amountof a rare earth metal compound and a promoting amount of rhenium.
 4. Ina process for the production of ethylene oxide wherein ethylene iscontacted in the vapor phase with an oxygen-containing gas at ethyleneoxide forming conditions at a temperature in the range of from about180° C. to about 330° C. in the presence of a silver metal-containingcatalyst, the improvement which comprises using a catalyst comprising acatalytically effective amount of silver, a promoting amount of alkalimetal, a promoting amount of a rare earth metal compound, a promotingamount of rhenium and a rhenium co-promoter selected from sulfur,molybdenum, tungsten, chromium and mixtures thereof supported on asuitable support having a surface area in the range of from about 0.05m² /g to about 10 m² /g.
 5. In a process for the production of ethyleneoxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising silver, an alkali metal promoter,from about 10 parts per million to about 1000 parts per million of rareearth promoter, expressed as the element, a rhenium promoter and arhenium co-promoter selected from sulfur, molybdenum, tungsten, chromiumand mixtures thereof supported on a porous refractory support having asurface area in the range of from about 0.05 m² /g to about 10 m² /g,wherein the rare earth is applied to the support in the form of an oxideor an oxygen-bound species.
 6. In a process for the production ofethylene oxide wherein ethylene is contacted in the vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at atemperature in the range of from about 180° C. to about 330° C. in thepresence of a silver metal-containing catalyst, the improvement whichcomprises using a catalyst comprising from about 1 percent by weight toabout 25 percent by weight of the total catalyst of the silvercompound(s), expressed as the metal, from about 10 parts per million toabout 3000 parts per million by weight of alkali metal compound(s),expressed as the metal, per million parts by weight of the totalcatalyst, from about 10 parts per million to about 1000 parts permillion by weight of the total catalyst of rare earth metal compound(s),expressed as the element, from about 0.1 micromoles to about 10micromoles per gram of total catalyst of rhenium compound(s), expressedas the metal, and from about 0.1 micromoles to about 10 micromoles pergram of total catalyst of rhenium co-promoter compound(s), expressed asthe element, to provide the catalyst with a catalytically effectiveamount of silver, a promoting amount of alkali metal, a promoting amountof a rare earth metal compound, a promoting amount of rhenium and apromoting amount of rhenium co-promoter.